Metal oxides have numerous uses. In particular, zinc oxide is used for diverse purposes including, for example, use as a white pigment, as a catalyst, as a constituent of antibacterial skin-protection ointment, and as an activator for rubber vulcanization. Sunscreens and wood varnishes contain finely divided zinc oxide as an ultraviolet (UV)-absorbing pigment.
Zinc oxide is useful as a UV-absorbing agent because it does not degrade upon prolonged exposure to UV light. When its particle size is less than about 20 nanometers (nm), however, its bandgap shifts to higher energy as its particle size decreases, due to quantum confinement. To maximize the number of UV wavelengths absorbed by zinc oxide, particles having a bandgap as close as possible to the semiconductor's bulk bandgap are desirable. Since the shift from the bandgap of the bulk material is greater the smaller the particle size, crystalline particle diameters of at least about 5 nm generally can be useful. Such particle diameters provide bandgap values relatively close to those of the bulk material, resulting in a relatively broad range of absorbed wavelengths.
Nanoparticles of zinc oxide can be sufficiently small, however, so as to scatter only negligible amounts of visible light. Thus, UV light absorbing, but visible light transparent, composites (for example, transparent organic-inorganic hybrid materials, plastics, paints and coatings) can be made using zinc oxide nanoparticles as a filler. To maintain optical transparency, particle diameters (and the diameters of any agglomerates present) generally should be less than about one-tenth the wavelength of light (for example, below about 30 nm).
The preparation of zinc oxide by both dry and wet processes is known. The classical dry method of burning zinc generates aggregated particles having a broad size distribution. Particularly finely divided zinc oxide is prepared predominantly by wet chemical methods using precipitation processes. Precipitation in aqueous solution generally gives hydroxide- and/or carbonate-containing materials that require thermal conversion to zinc oxide. The thermal post-treatment can have a negative effect on the finely divided nature of the particles, as the particles are subjected during this treatment to sintering processes that can lead to the formation of micrometer (μm)-sized aggregates. These aggregates can be broken down only incompletely to the primary particles by milling or grinding.
In non-aqueous solutions (or aqueous solutions above the decomposition temperature of zinc hydroxide), zinc oxide can be grown through a simple base precipitation according to the following equation (where X is generally a suitable anion and Y is a suitable cation):ZnX2+2YOH→ZnO+2YX+H2OParticle growth takes place through an Ostwald ripening process and is diffusion-dependent. As such, particle growth is rather slow at room temperature if 8 nm or larger diameter particles are desired. Elevating the reaction temperature can speed the process to reasonable rates, but this can simultaneously increase the rate of agglomeration.
Various common zinc salts (for example, zinc acetate) have been used as the starting salt in such non-aqueous precipitation processes. However, such starting salts have generally required the use of dilute solutions to avoid relatively high rates of agglomeration, and zinc oxide grown from such salts has tended to form agglomerates that are unsuitable for applications requiring transparency.
Other processes for the preparation of nanosize zinc oxide particles utilize expensive starting materials (for example, zinc alkoxides), require the use of emulsifiers, are complex, provide agglomerates, provide slow particle growth, provide insufficient control over particle size, and/or cannot provide often preferred particle sizes (for example, average primary particle diameters of about 5 to about 30 nm).